1. Field of the Invention
The present invention relates to a microscope system, and more particularly, to a microscope system comprising an objective lens and an actuator element for moving the objective lens in order to focus the objective lens with respect to a specimen to be examined.
2. Description of Related Art
An optical microscope serves to view a sample by placing it in the central optical axis of an objective lens and by establishing a suitable distance between sample and objective lens, such that an image of the specimen can be formed by the microscope optics, consisting of an objective lens or an objective lens in conjunction with a suitable tube lens. Observation of the resulting image is accomplished either directly by a detector placed in the intermediary image plane or by the eye of the operator, in which case, usually an additional optical element (i.e., an ocular lens) is needed.
Both detector means yield two-dimensional image information, i.e., only one section of a three-dimensional object is “in focus.” Given that microscope optics usually exhibit a high numerical aperture, this focused section is very narrow, and in order to construct a three-dimensional image one has to record a series of images at different focus positions. For this purpose, the object stage is moved relative to the objective lens, which is fixed in its z-position, or the objective lens is moved relative to the z-position fixed specimen. The latter type, called “fixed stage” microscopes, have the advantage that assemblies attached to the sample (e.g., micropipettes, perfusion devices, etc.) do not need to be moved simultaneously when the focus position is changed.
In order to allow for more than one magnification of the microscope, different objective lenses with different focal lengths are commonly employed. In prior art, they are mounted on a revolver, which allows the selected objective to be brought into the central optical axis of the microscope. This is done manually or in a motorized fashion. Fixed stage focusing then occurs by moving the whole objective revolver relative to the specimen. Manual research microscopes employ two drive mechanisms for this purpose, a coarse drive to bring the objective in position, and a fine drive for finding the precise focus position. In motorized versions, coarse and fine drive are usually united into one drive, and when an even finer positioning is required, piezo elements are placed between the objective revolver and individual objectives. However, such dedicated piezo drives extend the distance between the objective and the remaining microscope optics by 15-20 mm and exhibit several distinct disadvantages:    Piezo-drives are expensive. To drive n objectives, n piezo drives and n piezo drive electronics boards are required.    all objectives, which are not equipped with piezo drives, require spacers in order to have their focal plane at the same position.    The cables leading to each piezo drive are in the way when the objective revolver is turned in order to position another objective in the central optical axis of the system.    The distance between the objective and the residual optics of the microscope is affected by these spacers or the additional length of the piezo drives. While this has no detrimental impact in systems with infinity corrected optics, in which the position of the objective's back focal does not matter, it must be corrected for in systems like confocal or Total Internal Reflection Fluorescence (TIRF) arrangements.
The rotation axis of objective revolvers is usually inclined at an angle in order to allow the changing of an objective without having to withdraw it from its focused position. If the rotation axis were vertical, the objective being rotated out of the beam and the one being rotated into the beam would both touch the holder for the specimen during rotation. However, with motorized drives, which facilitate the re-finding of a previous position, one no longer needs to change objectives without withdrawing them. This allows the usage of schemes for holding objectives, moving them into the central optical axis of the microscope system and focusing them, which serve several other purposes at the same time as will be described below.
U.S. Patent Application Publication 2002/0001126 A1 discloses a microscope system comprising an inventory of a plurality of objective lenses, wherein one of the objective lenses is selected and is moved along a guide rail from a storage position to a reference position within the central optical axis of the microscope system. The reference position is defined by a retaining element. The guide rail is designed such that the objective lens moves—in the vicinity of the retaining element—substantially coaxially with the optical axis.